Membership with Userid as Uniqueidentifier joining to custom table

I have implemented the Membership tables for using ASP.NET's provided classes. It has created a table aspnet_Users with UserId as a primary key and the data type is uniqueidentifier.

 I've created my own custom table for user's notes which I would like to store the userId to identify who has created the note.  My database experience has told me that I need to retrieve the userId from the logged in user and store it in the database. So I tried something like this:

note.CreatedBy = user.ProviderUserKey

Obviously this isn't right. I am hoping some one can inform me how to get the userID to be recored in my notes table?
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